THE CORRELATION OF STRENGTH AND FORMALDEHYDE EMISSION WITH THE CRYSTALLINE AMORPHOUS STRUCTURE OF UF RESINS

Particle board of low formaldehyde emission and acceptable performance were prepared using a UF resin of U:F molar ratio 1:0.7 by a modification of the resin synthesis which could be described as the addition of a branching agent. Crystallographic studies showed that there is a strong process influence on the final crystallinity and performance of the finished resin. Thus, in the 1:0.5 to 1:1.1 U:F molar ratio range, crystallinity of the cured UF resins, when cured alone, increases with increasing NH4Cl hardener content and with increasing curing temperature. In general the more rapid the curing the higher the degree of crystallinity. It was also shown experimentally that a UF resin when cured in presence of wood, can only be amorphous, even at U:F molar ratios which favour crystallinity when the resin is cured alone. The 1:0.7 molar ratio resin prepared is amorphous when cured on wood and microcrystalline when cured by itself. Molecular mechanics studies support this by showing that the cohesion forces between UF oligomers and cellulose are in general stronger than the intermolecular forces of the resin. Molecular mechanics calculations show that the various packing configurations of UF oligomers are energetically favourable. The diffraction patterns of the UF resin can be attributed to regions of crystallinity in the resin corresponding to mixtures of these minimum energy configurations. Under certain conditions KCl salt is formed during the curing process and can be readily identified crystallographically.